A new school of thought: Micrometer-sized silver chloride (AgCl) particles (red) in deionized water move under UV illumination by self-diffusiophoresis (see picture). Each AgCl particle secretes ions to which the other particles respond, and they are observed to "school" into regions with higher concentrations of particles. Photo-inactive silica particles (blue) also respond to the chemical secretion by swimming towards and surrounding individual AgCl particles.